#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>

template <class T>
T gcd(T n, T m) {
    while (m != 0) {
        n %= m;
        std::swap(n, m);
    }
    return n;
}

void solve() {
    int a, b;
    std::cin >> a >> b;

    if (gcd(a, b) > 1) {
        std::cout << -1 << std::endl;
        return;
    }

    int m = a * b * 2;
    std::vector<bool> ok(m + 1, true);
    for (int x = 0; a * x <= m; ++x) {
        for (int y = 0; a * x + b * y <= m; ++y) {
            ok[a * x + b * y] = false;
        }
    }

    int ans = std::accumulate(ok.begin(), ok.end(), 0);
    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}